Louderback. — Benitoite. 



151 



trigonal pyramid and the corresponding trigonal prisms. The 

 normal angle between the basal plane and the pyramid is about 

 40° 14'. If the pyramid be taken as a unit pyramid of the first 

 order, this would yield an axial ratio of .7327. if of the second 

 order, .8460. 



The most common habit is pyramidal, one pyramid being the 

 chief form, the other occurring as a small but regular and bril- 

 liant truncation. One or both prisms may be present as narrow 

 truncations and also a small triangular basal plane. 



Occasionally the base is developed into a broad plane, the 

 crystals then having a more nearly tabular habit. The outline 

 of the base may then be hexagonal but the edges corresponding to 

 one pyramid will be considerably longer than the others. 



Only one crystal was found where the two pyramids were 

 nearly equally developed. The development of the faces at one 

 end of the principal axis always corresponds so well with those 

 at the other, that it gives the impression that the horizontal plane 

 of symmetry is present. No tendency towards a prismatic habit 

 was observed. The angles between two adjoining pyramid faces 

 at one end of axis is 68° 1'. There is an imperfect pyramidal 

 cleavage. The fracture is conehoidal to subconchoidal. The 

 hardness is 614-61/2 ; distinctly above orthoclase and labradorite 

 and below chrysolite and quartz; density, 3.64-3.65. 



The refractive index is quite high, which adds greatly to the 

 beauty of the cut stone. For the ordinary ray it is about 1.77 

 (sodium light), for the extraordinary, about 1.80. The double 

 refraction is therefore very strong and the mineral optically 

 positive. Basal sections show a perfect uniaxial cross which 

 gives a distinct positive reaction with the mica plate. The pleo- 

 ehroism has already been described and evidently the absorption 

 is e>o. Some difficulty was experienced in getting a value for e, 

 as sodium light is strongly absorbed even in light colored speci- 

 mens a couple of millimeters thick. 



The mineral fuses quietly to a transparent glass at about 3. 

 It is practically insoluble in hydrochloric acid, but it is quite 

 easily attacked by hydrofluoric acid, and dissolves readily in 

 fused sodium carbonate. 



The mineral has proved to be of considerable interest from 



